A venting valve loaded by elastic force is to be understood both as a venting valve that is movable and restorable by virtue of elastic deformation properties and is composed at least partially of an elastomer material, and as an elastic force applied by an external spring arrangement. A metering device of this kind is used for the metering of liquid media, gel media and powdered media, or combinations of these. The media can be intended in particular for pharmaceutical or cosmetic applications. The metering device permits dispensing of a predetermined quantity of the medium. An exact metering of the medium may be necessary for pharmaceutical applications.
Document DE 44 38 364 A1 describes a metering device in which a medium storage unit is closed by a pumping system designed as a thrust piston. The pumping system comprises a filling channel, which is also suitable as a venting channel for the medium storage unit. To allow ambient air to flow into the medium storage unit, for example for pressure equalization after medium has been discharged by the pumping system, a venting valve is provided which is designed as an annular seal. This seal at least partially surrounds the pumping system elastically and thus closes the venting channel between the medium storage unit and the environment. In the event of a pressure difference between the medium storage unit and the environment, for example in the event of an underpressure in the medium storage unit, the seal allows ambient air to flow into the medium storage unit by means of an elastic deformation. As soon as the pressure difference falls below a pressure level defined by the elastic properties of the seal, the seal closes the venting channel and thus prevents further admission of ambient air. This procedure takes place whenever a pressure difference exists that can cause an elastic deformation of the seal. This can also take place independently of an actuation of the metering device.
Particularly for pharmaceutical media, it is not only important to meter an exact quantity of the medium, but also to ascertain the amount of active substance metered with the corresponding quantity of medium. In known metering devices, the active substance concentration may very gradually change, particularly if readily volatile, evaporated constituents of the medium escape from the medium storage unit by way of the venting channel. In these circumstances, even with constant metering of the quantity of medium, a change can occur in the metered amount of active substance. A change of this kind in the amount of metered active substance is undesirable in pharmaceutical media.